The present invention relates to improving the sound quality of voice signals transmitted during a call between two digital wireless telephones. More particularly, the present invention relates to a method for diverting a wireless telephone call from a circuit switched network to a data network before the call is answered if the called party""s digital wireless telephone is able to support one of the voice compression algorithms supported by the calling party""s digital wireless telephone and if both digital wireless telephones have access to the same data network.
Currently, many telephone calls from one digital wireless party to another digital wireless party are processed through a circuit switched network such as the public switched telephone network (PSTN). One example of such a telephone call may be illustrated by analyzing the process of completing a telephone call from a wireless party on the east coast of the United States to a wireless party on the west coast.
First, the east coast wireless party dials the west coast wireless party""s telephone number using their digital wireless telephone""s keypad. When the number is dialed, the east coast party""s digital wireless telephone uses a line control signaling technique, such as American National Standards Institute-136 (ANSI-136) to form a call request message. The east coast party""s digital wireless telephone then transmits this line control signaling call request message through the air to a cell site (also referred to as a xe2x80x9cbase stationxe2x80x9d) which serves the geographic region where the east coast party""s wireless telephone is currently located and registered. This east coast cell site sends the line control signaling call request message to the east coast party""s line/network interface switch (referred to as a xe2x80x9cmobile switching centerxe2x80x9d (MSC) in the context of wireless communication systems) via dedicated circuits.
Next, the east coast MSC uses a network control signaling technique such as Integrated Digital Service Network User Part (ISUP) to reformat the line control signaling call request message into a circuit control signaling call request message and send this circuit control signaling call request message along a xe2x80x9csignaling pathxe2x80x9d of network switches within the PSTN to an MSC on the west coast which is assigned to the west coast party. As the circuit control signaling call request message travels along this xe2x80x9csignaling path,xe2x80x9d various and intermediate network switches contribute a xe2x80x9ctalkpathxe2x80x9d of circuits from the east coast MSC to the west coast MSC. The xe2x80x9ctalkpathxe2x80x9d will be utilized to send voice signals through the PSTN once the call set up is completed.
The west coast MSC, known as the west coast party""s home MSC, then queries a home location register (HLR) to determine the MSC currently nearest to the west coast party""s digital wireless telephone. If the response to the query indicates the west coast party has roamed to a geographic region covered by a west coast MSC other than the west coast party""s home MSC, the home MSC then delivers the circuit control signaling call request message to that MSC. The receiving MSC then continues the process of sending the circuit control signaling call request message to the west coast party. For the purpose of this example, assume the west coast party is currently within the geographic region covered by their home MSC and that the additional step of forwarding the circuit control signaling call request message from the west coast party""s home MSC to another west coast MSC is not required.
The west coast party""s home MSC uses ANSI-136 line control signaling to reformat the circuit control signaling call request message back to a line control signaling call request message and sends the line control signaling call request message (via dedicated circuits) to a cell site which serves the geographic region where the west coast party""s wireless telephone is currently located and registered. This west coast cell site then transmits the line control signaling call request message through the air to the west coast party""s wireless telephone. Once the west coast party answers the call to their wireless telephone, both parties can exchange voice information using the xe2x80x9ctalkpathxe2x80x9d of PSTN circuits for the duration of the call.
For example, when the east coast party begins speaking into a microphone attached to their wireless telephone, the east coast party""s analog voice signal is compressed by their wireless telephone using a xe2x80x9cvoice compression algorithmxe2x80x9d executed by the cellular telephone. Using a voice compression algorithm to compress the analog voice signal into a low bit rate digital representation conserves the limited bandwidth resources available to wireless communication systems. The east coast party""s digital wireless telephone then modulates a carrier frequency with the compressed voice signal and transmits the modulated signal through the air to the east coast cell site. The east coast cell site demodulates the received voice signal and sends the demodulated signal to the east coast MSC which uses a voice compression algorithm to decompress the received voice signal. (Alternatively, decompression may take place at the cell site.) Decompression is needed to make the voice signal compatible with the xe2x80x9ctalkpathxe2x80x9d of circuits. The east coast MSC then performs an intermediate encoding on the decompressed voice signal in accordance with a protocol such as G.711, and sends the G.711 encoded voice signal through the PSTN to the west coast party""s home MSC using the previously established xe2x80x9ctalkpath.xe2x80x9d
The west coast MSC decodes the G.711 encoded voice signal, compresses the voice signal (using a voice compression algorithm), and sends the compressed voice signal to the west coast cell site. (Alternatively, the compression may take place at the cell site.) The west coast cell site then modulates a carrier frequency with the compressed voice signal and transmits the modulated signal through the air to the intended west coast digital wireless telephone. The west coast digital wireless telephone demodulates the received voice signal and then uses a voice compression algorithm to decompress the received voice signal. The decompressed voice signal is then sent to the digital wireless telephone""s speaker for the west coast party to hear. Voice signals transmitted from the west coast wireless telephone party to the east coast wireless telephone party follow the reverse of the above steps.
Unfortunately, there are drawbacks to the above-described method of transmitting voice signals through the PSTN from one digital wireless telephone to another digital wireless telephone. First, the process of completing intermediate voice encoding slightly delays the transmission of the voice signals between the digital wireless telephones. Second, the process of completing intermediate voice encoding may either subject the voice signals to one or more compression/decompression cycles within the circuit switched network or subject the voice signals to voice xe2x80x9cenhancementsxe2x80x9d that otherwise serve analog line telephones much better than digital wireless telephones. Third, decompressing and compressing the same voice signal two or more times while transmitting that voice signal from the sender to the receiver degrades the quality of the voice signal. Finally, the cost of transporting voice signals through the PSTN can be relatively higher than transporting voice signals through a data network such as the Internet.
Thus, a need exists for a technique which solves the above-described problems.
The present invention provides a method for improving the sound quality of voice signals transmitted during a call between two digital wireless telephones by diverting the call from a circuit switched network to a data network before the call is answered. In accordance with the present invention, a call setup procedure using the circuit switched network is modified so that the calling party may determine the following information: 1) whether the called party""s digital wireless telephone is able to support one of the voice compression algorithms supported by the calling party""s digital wireless telephone and 2) whether both wireless telephones have access to the same data network. If the called party""s digital wireless telephone is able to support one of the voice compression algorithms supported by the calling party""s digital wireless telephone and both wireless telephones have access to the same data network, the circuit switched network call setup procedure is abandoned before the called party answers the call and the call is completed using the data network.